Hey Brian thanks for the edits on my Reds article. I noticed you mentioned you were unfamiliar with the term "age 21 season." It's actually a term I became familiar with from Brian Kenny. He uses it pretty regularly actually and it's used to show how old the player was for the majority of the season in question.

For example, Brandon Phillips' birthday is June 28, 1981. Although he starts the season at 31 years old, it'll be considered his age 32 season.